The present invention relates to a method of, and apparatus for, analysing signals in order to identify non-payload data elements in an incoming data stream. The method is especially suitable for use in rapidly identifying framing elements in a framed test signal, which enables efficient and effective measurement of the length of any service disruptions.
Telecommunications networks send large amounts of data, including voice communications, throughout the world. Since the delivery of this data is very important the traffic channels which transmit this data are frequently backed up with one or more protection channels, one of which takes over the transmission of data if one of a number of traffic channels fails. When a failure is detected in a channel, (for example when a cable is damaged) the data loss or degradation is detected at the receiving end, and a message sent back to the sending end to switch the transmission of the channel""s data to a protection channel.
New equipment must act at defined speeds to detect a data loss, switch to the protection channel and re-establish data transmission. In order to ensure that the equipment complies with required criteria, the time taken to re-establish transmission in the event of failure must be measured. Telecommunications test sets are used to test new links before they are made live. Amongst the many tests carried out is a measurement of the time it takes to perform a protection switch. This measures the time that the service is disrupted and is termed a xe2x80x9cservice disruptionxe2x80x9d test.
Known methods of service disruption measurement include use of a Pseudo-Random Binary Sequence (PRBS) which is sent as a test signal from the sending end. A delayed PRBS is generated by receiving equipment at the receiving end of the test apparatus and compared with the incoming signal. When data is being reliably transferred, the PRBS generated at the receiving end matches the incoming signal and no mismatches are detected. When a protection switch is induced, the delayed PRBS fails to match the incoming signal and a time counter is started. The receiving equipment then continuously seeds its PRBS generator with the incoming (faulty) signal and looks for mismatches between the incoming signal and the delayed PRBS. When a mismatch is detected, the receiving equipment re-seeds its PRBS generator and the process repeats until the generated sequence again matches the incoming signal. If the match persists for an accepted, predetermined time (e.g. 200 ms) then the protection switch time is taken as the time from the initial mismatch until the last error was detected. In order to record accurately the time that the incoming signal becomes fault-free and matching is re-established, the value of the time counter is latched every time the receiving equipment re-seeds its PRBS generator or detects a mismatch. Thus after the required mismatch-free time has elapsed the latch contains the time of the duration of the service disruption.
Whilst a raw PRBS patterns which may be regarded as the payload data, can sometimes be sent through a telecommunications network it is often necessary to include frame alignment information in a test signal in order to allow the equipment being tested to function. The signal is divided into frames and each frame comprises a small number of framing and housekeeping bits and a relatively large number of payload data bits. This introduces a problem into the service disruption measurement in that when the test signal is once again received by the receiving end after a disruption it will not be known which bits are payload data bits and which are framing/housekeeping bits. If a non-payload data bit (such as a housekeeping or framing bit is taken as a PRBS payload data bit, a PRBS mismatch will occur and the receiving equipment will not accurately detect when the incoming signal has been restored.
Prior art methods and apparatus have provided several ways to effect framing of the incoming signal.
For ITU E1-E4 rates the framing sequence is a group of 8 to 12 bits at the start of a 125 xcexcs frame. The housekeeping bit follows immediately so that all the non-payload data bits are grouped together. Framing circuits in the receiving equipment detect a first framing bit sequence in a first frame and then look for another in the same position in the next frame. Using this technique, a frame will generally be found within a small number of frame periods. Thus the service disruption measurement can be made with a relatively short framing time. Once reception of the test signal is re-established and the framing of the signal is determined, the payload PRBS can be examined for the existence of errors. The receiving equipment interprets the error-free condition as indicating restoration of the incoming data signal, and records the time elapsed since the beginning of the disruption.
At ANSI rates of T1 and T3 (DS1 and DS3) the situation is somewhat different. In this case the framing and housekeeping bits are evenly spaced throughout the payload data. In the case of T1 they are 193 bits apart, and in the case of T3 the framing/housekeeping bits 85 bits apart.
Framing is generally performed by selecting a bit, and a corresponding later bit where one would expect the next framing bit to occur (i.e. 170 or 386 bits later) if the initially selected bit were a framing bit. If the later bit does not match the framing sequence, then the initially selected bit was not a framing bit, so the next bit of the incoming signal is selected and the test is repeated.
The time taken to detect frame alignment with circuits of this nature can be many tens or even hundreds of milliseconds. Only when the framing sequence is detected can payload analysis, and then any service disruption measurement, take place.
The order of magnitude of time for operation of a protection switch is a few milliseconds up to a typical maximum of around 50 ms. The framing to allow analysis of the payload must be faster than this in order to allow the above technique for measuring service disruption to be used effectively.
It is therefore desirable to identify the framing sequence, by identifying the non-payload data elements, more quickly and efficiently.
According to a first aspect of the present invention there is provided a method in accordance with claim 1.
According to a second aspect of the present invention there is provided apparatus in accordance with claim 17.